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Black scurf: how to recognise and treat it

Disease

Black scurf — Photo: Jerzy Opioła, CC BY-SA 4.0, Wikimedia Commons

Symptoms

Hard, irregular black crusts (sclerotia) on the tuber skin that look like dirt but do not wash off; the flesh underneath is sound and they peel away.
Reddish-brown sunken cankers on underground stems and stolons; sprouts can be killed before they emerge, leaving gaps in the row.
Upper leaves roll upwards and may turn purplish; small aerial tubers can form on the stems.
Misshapen tubers and fewer of them where stolons are girdled.

Organic treatment

Dig 3–5 cm of compost into the soil, which tends to reduce black scurf.
Lift the crop as soon as it is mature — scurf increases quickly once the tops die back.
Clear plant debris after harvest, as the fungus overwinters on it; hot-compost or remove it.

Chemical treatment

Seed-tuber treatment with fludioxonil and an in-furrow application of azoxystrobin at planting, where approved for home use; they reduce the disease but help little in heavily infested soil. Follow the label.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Prevention

Plant certified seed potatoes without black crusts on the skin; no varieties are resistant.
Plant into warm soil (above 8 °C), not too deep, and earth up gradually; warming seed tubers before planting speeds emergence.
Do not grow potatoes in the same place for at least 3 years and dig out self-sown potatoes.
Do not overwater early in the season — cold, wet soil favours the fungus.

Plants it affects

Potato

Sources

Figures checked against these sources.